[PATCH 2/3] sched_clock: widen the max and min time

[Steven Rostedt <rostedt@goodmis.org>]

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With keeping the max and min sched time within one jiffy of the gtod clock
was too tight. Just before a schedule tick the max could easily be hit, as
well as just after a schedule_tick the min could be hit. This caused the
clock to jump around by a jiffy.

This patch widens the minimum to
   last gtod + (delta_jiffies ? delta_jiffies - 1 : 0) * TICK_NSECS

and the maximum to
    last gtod + (2 + delta_jiffies) * TICK_NSECS

This keeps the minum to gtod or if one jiffy less than delta jiffies
and the maxim 2 jiffies ahead of gtod. This may cause unstable TSCs to be
a bit more sporadic, but it helps keep a clock with a stable TSC working well.

Signed-off-by: Steven Rostedt <srostedt@redhat.com>
---
 kernel/sched_clock.c |   11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

Index: linux-tip.git/kernel/sched_clock.c
===================================================================
--- linux-tip.git.orig/kernel/sched_clock.c	2008-07-06 22:35:35.000000000 -0400
+++ linux-tip.git/kernel/sched_clock.c	2008-07-06 22:33:49.000000000 -0400
@@ -96,14 +96,21 @@ static void __update_sched_clock(struct 
 	s64 delta = now - scd->prev_raw;
 
 	WARN_ON_ONCE(!irqs_disabled());
-	min_clock = scd->tick_gtod + delta_jiffies * TICK_NSEC;
+
+	min_clock = scd->tick_gtod +
+		(delta_jiffies ? delta_jiffies - 1 : 0) * TICK_NSEC;
 
 	if (unlikely(delta < 0)) {
 		clock++;
 		goto out;
 	}
 
-	max_clock = min_clock + TICK_NSEC;
+	/*
+	 * The clock must stay within a jiffie of the gtod.
+	 * But since we may be at the start of a jiffy or the end of one
+	 * we add another jiffy buffer.
+	 */
+	max_clock = scd->tick_gtod + (2 + delta_jiffies) * TICK_NSEC;
 
 	if (unlikely(clock + delta > max_clock)) {
 		if (clock < max_clock)

--